Update Readers Python API

1. Combine 'open_files', 'multi_pass_reader' and 'threaded_reader'
together to make the new 'open_files' interface.
2. Add some docstring.
3. Simplify interface names of 'create_XXX_reader', e.g, rename
'create_double_buffer_reader' to 'double_buffer'.

python/paddle/fluid/layers/io.py

@@ -22,7 +22,7 @@ from ..executor import global_scope
 __all__ = [
     'data', 'BlockGuardServ', 'ListenAndServ', 'Send', 'open_recordio_file',
     'open_files', 'read_file', 'create_shuffle_reader',
-    'create_double_buffer_reader', 'create_multi_pass_reader'
+    'create_double_buffer_reader'
 ]
 
 
@@ -283,7 +283,43 @@ def _copy_reader_create_op_(block, op):
     return new_op
 
 
-def open_recordio_file(filename, shapes, lod_levels, dtypes):
+def open_recordio_file(filename,
+                       shapes,
+                       lod_levels,
+                       dtypes,
+                       pass_num=1,
+                       for_parallel=False):
+    """
+    Open a RecordIO file
+
+    This layer takes a RecordIO file to read from and returns a Reader Variable.
+    Via the Reader Variable, we can get data from the given RecordIO file.
+
+    Args:
+       filename(str): The RecordIO file's name.
+       shapes(list): List of tuples which declaring data shapes.
+       lod_levels(list): List of ints which declaring data lod_level.
+       dtypes(list): List of strs which declaring data type.
+       pass_num(int): Number of passes to run. After completing the
+            given number of passes, 'has_next()' will return False.
+       for_parallel(Bool): Set it as True if you are going to run
+            subsequent operators in parallel.
+
+    Returns:
+       Variable: A Reader Variable via which we can get RecordIO file data.
+
+    Examples:
+       .. code-block:: python
+
+         reader = fluid.layers.io.open_recordio_file(
+                                          filename='./data.recordio',
+                                          shapes=[(3,224,224), (1)],
+                                          lod_levels=[0, 0],
+                                          dtypes=['float32', 'int64'])
+
+         # Via the reader, we can use 'read_file' layer to get data:
+         image, label = fluid.layers.read_file(reader)
+    """
     dtypes = [convert_np_dtype_to_dtype_(dt) for dt in dtypes]
     shape_concat = []
     ranks = []
@@ -310,6 +346,13 @@ def open_recordio_file(filename, shapes, lod_levels, dtypes):
     startup_var.persistable = True
     main_prog_var = _copy_reader_var_(default_main_program().current_block(),
                                       startup_var)
+
+    if pass_num > 1:
+        main_prog_var = multi_pass(reader=main_prog_var, pass_num=pass_num)
+
+    if for_parallel:
+        main_prog_var = for_parallel(reader=main_prog_var)
+
     return monkey_patch_reader_methods(main_prog_var)
 
 
@@ -318,11 +361,15 @@ def open_files(filenames,
                lod_levels,
                dtypes,
                thread_num,
-               buffer_size=None):
+               buffer_size=None,
+               pass_num=1,
+               for_parallel=False):
     """
     Open files
 
-    This layer takes a list of files to read from and returns a Reader Variable. Via the Reader Variable, we can get data from given files.
+    This layer takes a list of files to read from and returns a Reader Variable. 
+    Via the Reader Variable, we can get data from given files. All files must 
+    have name suffixs to indicate their formats, e.g., '*.recordio'. 
 
     Args:
        filenames(list): The list of file names.
@@ -331,6 +378,10 @@ def open_files(filenames,
        dtypes(list): List of strs which declaring data type.
        thread_num(int): The maximal concurrent prefetch thread number.
        buffer_size(int): The size of prefetch buffer.
+       pass_num(int): Number of passes to run. After completing the 
+            given number of passes, 'has_next()' will return False.
+       for_parallel(Bool): Set it as True if you are going to run 
+            subsequent operators in parallel.
 
     Returns:
        Variable: A Reader Variable via which we can get file data.
@@ -338,7 +389,7 @@ def open_files(filenames,
     Examples:
        .. code-block:: python
 
-         reader = fluid.layers.open_files(filenames=['./data1.recordio',
+         reader = fluid.layers.io.open_files(filenames=['./data1.recordio',
                                                      './data2.recordio'],
                                              shapes=[(3,224,224), (1)],
                                              lod_levels=[0, 0],
@@ -347,7 +398,7 @@ def open_files(filenames,
                                              buffer_size=2)
 
          # Via the reader, we can use 'read_file' layer to get data:
-         image, label = fluid.layers.read_file(reader)
+         image, label = fluid.layers.io.read_file(reader)
     """
     if buffer_size is None:
         buffer_size = thread_num
@@ -361,13 +412,12 @@ def open_files(filenames,
         shape_concat.extend(shape)
         ranks.append(len(shape))
 
-    var_name = unique_name('multiple_reader')
-
+    multi_file_reader_name = unique_name('multi_file_reader')
     startup_blk = default_startup_program().current_block()
-    startup_var = startup_blk.create_var(name=var_name)
+    startup_reader = startup_blk.create_var(name=multi_file_reader_name)
     startup_blk.append_op(
         type='open_files',
-        outputs={'Out': [startup_var]},
+        outputs={'Out': [startup_reader]},
         attrs={
             'shape_concat': shape_concat,
             'lod_levels': lod_levels,
@@ -377,14 +427,21 @@ def open_files(filenames,
             'buffer_size': buffer_size
         })
 
-    startup_var.desc.set_dtypes(dtypes)
-    startup_var.persistable = True
-    main_prog_var = _copy_reader_var_(default_main_program().current_block(),
-                                      startup_var)
-    return monkey_patch_reader_methods(main_prog_var)
+    startup_reader.desc.set_dtypes(dtypes)
+    startup_reader.persistable = True
+    main_prog_reader = _copy_reader_var_(default_main_program().current_block(),
+                                         startup_reader)
+    if pass_num > 1:
+        main_prog_reader = multi_pass(
+            reader=main_prog_reader, pass_num=pass_num)
 
+    if for_parallel:
+        main_prog_reader = for_parallel(reader=main_prog_reader)
 
-def __create_decorated_reader__(op_type, reader, attrs):
+    return monkey_patch_reader_methods(main_prog_reader)
+
+
+def __create_decorated_reader__(op_type, reader, attrs={}):
     var_name = unique_name(op_type)
     startup_blk = default_startup_program().current_block()
     startup_var = startup_blk.create_var(name=var_name)
@@ -400,12 +457,12 @@ def __create_decorated_reader__(op_type, reader, attrs):
     return monkey_patch_reader_methods(main_prog_var)
 
 
-def create_shuffle_reader(reader, buffer_size):
+def shuffle(reader, buffer_size):
     return __create_decorated_reader__('create_shuffle_reader', reader,
                                        {'buffer_size': int(buffer_size)})
 
 
-def create_double_buffer_reader(reader, place=None):
+def double_buffer(reader, place=None):
     attrs = dict()
     if place is not None:
         attrs['place'] = str(place).upper()
@@ -413,11 +470,15 @@ def create_double_buffer_reader(reader, place=None):
                                        attrs)
 
 
-def create_multi_pass_reader(reader, pass_num):
+def multi_pass(reader, pass_num):
     return __create_decorated_reader__('create_multi_pass_reader', reader,
                                        {'pass_num': int(pass_num)})
 
 
+def for_parallel(reader):
+    return __create_decorated_reader__('create_threaded_reader', reader)
+
+
 def read_file(file_obj):
     helper = LayerHelper('read_file')
     out = [